Dissolved hydrogen analyzer

ABSTRACT

The present invention provides apparatuses and processes for the measurement of hydrogen in aqueous solution at concentrations as low as about 0.1 nM. The present invention is capable of accurately and reproducibly measuring the concentration of dissolved hydrogen in an aqueous solution that also contains other dissolved gases, such as oxygen, carbon monoxide and sulfur compounds, such as hydrogen sulfide. In a presently preferred embodiment of a hydrogen analyzer  38  of the present invention, water containing dissolved hydrogen is equilibrated with a carrier gas by means of gas flow through a mass transfer device  10.  Carrier gas is equilibrated with hydrogen from the water within a gas equilibration volume  4  and is then circulated, by means of a pump  1,  through a circuit  14  that includes a moisture removal component  16,  an oxygen removal component  15  and a heated carbon monoxide and sulfur compound removal component  17,  which remove water, oxygen, carbon monoxide and sulfur compounds from the carrier gas without consuming or producing hydrogen. A sensor  7  measures the amount of hydrogen in the carrier gas from which moisture, oxygen, carbon monoxide and sulfur compounds have been removed.

[0001] The U.S. Government may have certain rights in this invention asprovided for in SBIR Contract No. F41624-97-C-0005 awarded by theDepartment of the Air Force.

FIELD OF THE INVENTION

[0002] The present invention relates to measurement of dissolvedmolecular hydrogen. In particular, the present invention relates to anapparatus and process for the measurement of hydrogen in water atconcentrations as low as on the order of 0.1 nM.

BACKGROUND OF THE INVENTION

[0003] Molecular hydrogen present in water in a dissolved form(dissolved hydrogen) is an important indicator of various biological andchemical processes. These processes include in situ bioremediation ofgroundwater by engineered methods or by natural attenuation, anaerobicreactors for waste treatment including anaerobic digesters, anaerobicbioprocesses for the manufacture of biochemicals, includingfermentation, operation of subsurface, permeable metal-reactive wallsfor remediation of chlorinated chemicals in groundwater by reductivedehalogenation and corrosion of metals in process systems includingboilers. Dissolved hydrogen can be an indicator of the nature, extent,or stability of these processes.

[0004] The concentration of dissolved hydrogen can be extremely low. Forexample, one class of anaerobic bacteria known as iron-reducing bacteriatypically demonstrate dissolved hydrogen concentrations in groundwaterin the range of 0.1 to 1.0 nM when at steady state (F. H. Chapelle andP. B. McMahon, J. Hydrology, 127:85-108 (1991)).

[0005] Methods available for measurement of dissolved hydrogen involvedirect measurement in the liquid of interest or extraction of dissolvedhydrogen into a carrier gas which is then analyzed. Only one methodexists for measuring dissolved hydrogen concentrations as low as 0.1 nMand is called the “bubble strip” method (F. H. Chapelle and P. B.McMahon, J. Hydrology, 127:85-108 (1991)). This method involvesequilibration of a bubble of nitrogen with a flowing stream ofgroundwater in a gas sampling bulb made from glass. Samples of the gasbubble are injected into a reduction gas analyzer over time until thegas bubble is in equilibrium with the groundwater. The reduction gasanalyzer employs chemical reduction of a heated bed of mercuric oxide byhydrogen to form gaseous mercury that is sensed by an ultravioletdetector. Chromatographic separation of hydrogen from other reducinggases is required prior to mercuric oxide reduction. The gaseoushydrogen concentration is then related to the dissolved hydrogenconcentration by Henry's law where 0.1 nM dissolved hydrogenapproximately correlates to 0.125 ppm of gaseous hydrogen at equilibriumand ambient temperature and pressure. This method is difficult,time-consuming, and expensive to use and has therefore not gainedwidespread acceptance as an analytical method. Application of thereduction gas analyzer, in combination with hydrogen equilibration overTeflon tubing, to anaerobic digestion in particular is cited as beinglimited because of its “sophistication, high cost, detection limits, andinterference from other solutes” (K. Kuroda, R. G. Silveira, N. Nishio.H. Sunahara, and S. Nagai, J. Ferment. Boeing, 71:418-423 (1991)). Thegas bubble equilibration method also is greatly subject to operatorerror, in part because of mass transfer limitations. A. Pauss, G. Andre,M. Perrier, and S. R. Guiot, Appl. Environ. Microbiol., 56:1636-1644(1990). Other methods that are available for hydrogen measurement areinsensitive at these low concentrations and in environments of interest.

[0006] A Clark probe with reversed polarity is capable of hydrogenmeasurement in gases or liquids. The lower detection limit is 500 ppm ingases (F. J. Hanus, K. R. Carter, and H. J. Evans, Methods inEnzymology, 69:731-739 (1980)) and 15 μg/L (7,500 nM) in water (J. D.Istok, M. D. Humphrey, M. H. Schroth, M. R. Hyman, and K. T. O'Reilly,Ground Water, 35:619-631 (1997)). Another electrochemical probe fordissolved hydrogen described by Strong (G.E. Strong and R. Cord-Ruwisch,Biotechnol. Boeing, 45:63-68 (1995)) has a detection limit of 30 Papartial pressure which is equivalent to 240 nM. Ozawa et al. in EP0096417A1 describe an electrochemical hydrogen sensor that has asensitivity of 500 nM dissolved hydrogen. Kitamura et al. in EP0122511A2describe a similar electrochemical hydrogen sensor that compensates foroxygen but does not remove its influence and has an insufficientsensitivity in the nM range. Other electrochemical methods employingfuel cells have been described with detection limits of 1 μM (1,000 nM)(J. -P. Gebeault, J. Van Berlo, and M. Dymarski, Trans. Amer. Nucl.Soc., 46:612-613 (1984)) and 80 nM. A. Pauss, R. Samson, S. Guiot and C.Beauchemin, Biotechnol. Bioeng., 35:492-501 (1990). Hydrogen sulfide andoxygen interfere with the performance of these probes. A. Pauss, R.Samson, S. Guiot and C. Beauchemin, Biotechnol. Bioeng, 35:492-501(1990). In one case, oxygen did not interfere as long as it was presentin lower concentrations than hydrogen (N. Hara and D. D. Macdonald, J.Electrochem. Soc., 144:4152-4157 (1997)). In the practice of the presentinvention, very low hydrogen concentrations render this requirementimpractical.

[0007] Gas chromatography with thermal conductivity detection can beused to detect hydrogen in gases. This method can be used to detect 0.5umoles of injected hydrogen (F. J. Hanus, K. R. Carter, and H. J. Evans,Methods in Enzymology, 69:731-739 (1980)) which, based on a 1-mlinjection, translates to a concentration of 12 ppm in gas or anequilibrium dissolved concentration of 9.6 nM.

[0008] An instrument based on thermal conductivity has been developed tomeasure hydrogen in steam or hydrogen dissolved in water and has aninadequate detection limit of 100 nM (C. R. Wilson, Electric PowerResearch Institute Report NP-2650 (1982)).

[0009] Equilibration of dissolved hydrogen in water with a carrier gasfollowed by removal of coexisting gases (e.g., oxygen, hydrogen sulfide,carbon dioxide) that can interfere with or dilute hydrogen duringanalysis has been attempted but not at sufficiently low detectionlimits. Removal of carbon dioxide from carrier gas equilibrated withrumen fluid followed by gas chromatography resulted in a detection limitof 10 nM (J. A. Robinson, R. F. Strayer, and J. M. Tiedje, Appl.Environ. Microbiol., 41:545-548 (1981)). This method is not applicablewhere carbon dioxide is present in low concentrations.

[0010] Mass spectrometry can be used to detect hydrogen in gases or, viause of a membrane system, in liquids (P. Dornseiffer, B. Meyer, and E.Heinzle, Biotechnol. Bioeng., 45:219-228 (1995)). Hydrogenconcentrations detected in liquids are in the low μM (1,000 nM) rangeand accurate measurement can be compromised by biofilm growth on themembrane surface which requires periodic maintenance and cleaning.

[0011] A palladium-coated micromirror fiber optic sensor developed bySandia National Laboratories was shown to be capable of sensing 50 ppmof hydrogen in transformer oil (M. A. Butler, R. Sanchez, and G. R.Dulleck, Sandia Report SAND96-1133.UC-706 (1996)).

[0012] Various types of solid state sensors are capable of hydrogendetection. Keithley (Cleveland, Ohio) sells a hot wire semiconductortype sensor named CH-H. This sensor contains a platinum wire in asintered tin oxide semiconductor bead. Hydrogen reacts with oxygen onthe platinum wire thereby generating heat. The altered resistance of theplatinum wire is sensed in a bridge circuit. This sensor requires thepresence of oxygen and is sensitive to approximately 10 ppm hydrogen ingas or an equilibrium dissolved concentration of 8 nM.

[0013] Sensors based on the observed change in the electrical resistanceof platinum and palladium upon adsorption of hydrogen have beendescribed. These sensors can be immersed in water but have a detectionlimit of 5,000 nM dissolved hydrogen (C. Liu and D. D. Macdonald, J.Supercritical Fluids., 8:263-270 (1995)).

[0014] Lundstrom described metal oxide semiconductor (MOS) transistorscontaining a palladium gate (K. I. Lundstrom, M. S. Shivaraman, and C.M. Svensson, J. Appl. Physics., 46:3876-3880 (1975); I. Lundstrom,Sensors and Actuators., 1:403-426 (1981)). The sensitivity of thesestructures to hydrogen in gas is highly dependent on oxygenconcentration. A 10 mV response was observed with 0.5 ppm hydrogen inair and with 0.03 ppb hydrogen in an inert gas such as argon ornitrogen. The difference in response is due to the oxygen content ofair. These sensors are also sensitive to hydrogen sulfide albeit atten-fold greater concentrations than hydrogen (I. Lundstrom, Sensors andActuators., 1:403-426 (1981)) and sulfur compounds are well known fortheir poisoning of metallic surfaces. A hydrogen leak detector based onsuch MOS sensors demonstrated a practical sensitivity of 1 ppm (L.Stiblert and C. Svensson, Rev. Sci. Instrum., 46:1206-1208 (1975)).

[0015] A hydrogen sensor with a practical sensitivity of 1 ppm in gas isdescribed by Hughes et al. in U.S. Pat. No. 5,279,795. This type ofsensor is disadvantageous in part because of the slow response at lowhydrogen concentrations. The sensitivity of this sensor is negativelyaffected by the presence of oxygen. It was reported that hydrogensulfide does not poison the sensor; however, the tests were conducted inair where hydrogen sulfide poisoning is known to be mitigated byoxidation. This sensor has been incorporated into a hand held detectorby DCH Technology which has a detection limit of 10 ppm in gas or anequilibrium dissolved concentration of 8 nM.

[0016] Immersion of MOS devices in anaerobic water is not practicalbecause of incompatibility. Protection of a MOS device with agas-permeable membrane such as Goretex™ would be expected to work fordetection of dissolved hydrogen in anaerobic water but does not. Whileanaerobic conditions in groundwater would seem to imply the absence ofoxygen; in fact, oxygen is often observed in “anaerobic” groundwater,presumably due to the heterogeneous nature of many aquifers.Additionally, MOS devices are poisoned by hydrogen sulfide. Hydrogensulfide is a common contaminant present in anaerobic groundwater and inanaerobic digesters. These sensors are also inhibited by carbon monoxidewhich is found in anaerobic environments.

[0017] Neuwelt in U.S. Pat. No. 3,661,010 describes a method employingan electrochemical sensor covered by a membrane over which flows theliquid. This method is disadvantageous because no method for removal ofinterferences is provided and insufficient sensitivity exists. Adissolved hydrogen analyzer manufactured by Orbisphere Laboratories(Inverness, Calif.) also uses an electrochemical sensor covered by amembrane but is sensitive only to 15 nM dissolved hydrogen and thissensitivity is adversely affected by oxygen.

[0018] Immersion of any type of hydrogen probe in a biological mediumcan also result in growth of biofilm on the probe. Such biofilm growthcan subsequently result in dissolved hydrogen consumption or productionwhich can affect the measurement accuracy. Such effects were observedwith a gas diffusion probe used in conjunction with a reduction gasanalyzer (H. Kramer and R. Conrad, FEMS Microbiol. Ecol., 12:149-158(1993)).

[0019] Schuy in U.S. Pat. No. 3,920,396 describes a membraneequilibration device that uses an extraction gas circulating in a closedloop to attain equilibrium between the gas and liquid sample of fixedvolume. This method is disadvantageous because dissolved gases with highHenry constants will be predominately stripped into the gas phase, andthe attained equilibrium will occur at a dissolved gas concentrationthat is significantly less than the original dissolved gasconcentration. Furthermore, this method provides no means for removal ofinterfering gases that also equilibrate across the membrane.

[0020] Baillie et al. in U.S. Pat. No. 4,916,079 describe a gas-liquidequilibration device that uses a constant flow of liquid which overcomesthe disadvantages of U.S. Pat. No. 3,920,396 by using a continuous flowof liquid and spiking the equilibration gas with a known quantity of theanalyte to overcome interferences. This method is not applicable to theanalysis of low levels of hydrogen in the practice of the presentinvention because the concentrations of hydrogen are too low relative tothe concentrations of interfering gases.

[0021] Ketchum et al. in U.S. Pat. No. 4,236,404 describe a device tomonitor hydrogen and other gases in electrical insulating liquids suchas transformer oils that employs equilibration between gas and liquidand a thermal conductivity detection gas chromatography for analysis.This device overcomes interferences by chromatographic separation butdoes not have sufficient sensitivity for the low-concentrationapplications contemplated by this invention.

[0022] Thus, to the best of applicant's knowledge no practical devicecapable of detecting concentrations on the order of 0.1 nM dissolvedhydrogen exists with the sole exception of the reduction gas analyzerwhich is expensive and must be used in combination with the bubble stripmethod which is difficult to use.

SUMMARY OF THE INVENTION

[0023] The present invention provides apparatuses and processes for themeasurement of hydrogen in aqueous solution at concentrations of lessthan 1.0 nM, and preferably as low as about 0.1 nM. The presentinvention is capable of accurately and reproducibly measuring theconcentration of dissolved hydrogen in an aqueous solution that may alsocontain other dissolved gases, such as oxygen, carbon monoxide andsulfur compounds, such as hydrogen sulfide.

[0024] In one aspect, the present invention provides a hydrogen analyzerthat is capable of accurately measuring the amount of hydrogen inaqueous solution at concentrations of less than 1.0 nM, and preferablyas low as about 0.1 nM. The hydrogen analyzer includes a mass transferdevice, having an aqueous portion through which passes an aqueousanalyte, such as contaminated ground water, and a gaseous portion,through which passes a carrier gas, such as nitrogen gas. Within themass transfer device, hydrogen gas is transferred from the aqueousanalyte to the carrier gas. The hydrogen analyzer also preferablyincludes a gas equilibrium volume within which hydrogen gas transferredfrom the aqueous analyte is equilibrated with the carrier gas. Thehydrogen analyzer also preferably includes a component for removal ofcarbon monoxide from the carrier gas containing hydrogen; a componentfor removal of sulfur compounds from the carrier gas containinghydrogen; a component for removal of oxygen from the carrier gascontaining hydrogen; and a hydrogen sensor for measuring the amount ofhydrogen in the carrier gas. Preferably the carbon monoxide removalcomponent and the sulfur compound removal component are unitary, i.e.,are not physically separate components. Preferably the hydrogen analyzeralso includes a water removal component. The foregoing components of thehydrogen analyzer of the present invention are in gaseous communication,for example by means of tubes or pipes, and a pump moves the carrier gasthrough the components of the hydrogen analyzer.

[0025] Carbon monoxide removal is necessitated because of the generationof carbon monoxide during the removal of oxygen in accordance with thepreferred method of the present invention. Alternative methods of oxygenscavenging may be adapted for use in the present invention, however,which do not generate carbon monoxide, in which case a carbon monoxideremoval component is not required. Likewise sulfur and water removal areonly necessitated when present in the carrier gas.

[0026] In a presently preferred embodiment of the hydrogen analyzer ofthe present invention, water containing dissolved hydrogen isequilibrated with a carrier gas by means of gas flow through the masstransfer device. Equilibrated carrier gas within the gas equilibrationvolume is then circulated, by means of the pump, through a circuit thatincludes the moisture removal component, the oxygen removal componentand a heated carbon monoxide and sulfur compound removal component,which remove water, oxygen, carbon monoxide and sulfur compounds fromthe carrier gas without consuming or producing hydrogen. Preferably themoisture removal cartridge is located before the carbon monoxide andsulfur compound removal cartridge in the gas flow path. A sensormeasures the amount of hydrogen in the carrier gas from which moisture,oxygen, carbon monoxide and sulfur compounds have been removed.

[0027] The mass transfer device can be any device that allowsequilibration of dissolved hydrogen with the carrier gas phase. Examplesof acceptable mass transfer devices include hollow fiber gas transfermodules and sparging devices. Presently preferred moisture-removalcompositions, for inclusion in the moisture removal component, aremolecular sieves and calcium sulfate compositions.

[0028] Presently preferred carbon monoxide removal compositions, forinclusion in the carbon monoxide removal component, are metal oxidecatalysts including those based on copper and zinc, such as the finelydispersed cupric oxide catalyst named R3-11, manufactured by BASF(Parsipany, N.J.). Preferably the carbon monoxide removal compositionincludes a colorimetric carbon monoxide indicator. The carbon monoxideremoval component also preferably includes a heater for heating thecarbon monoxide removal composition within. Preferably the carbonmonoxide composition is also capable of adsorbing sulfur compounds. Apresently preferred carbon monoxide removal composition that is alsocapable of adsorbing sulfur compounds is catalyst R3-11.

[0029] The oxygen removal component can contain any composition that iscapable of efficiently and rapidly removing oxygen from the carrier gaswithout producing or consuming hydrogen. Presently preferred oxygenremoval compositions are based on ascorbic acid.

[0030] Presently preferred hydrogen-sensing components include metaloxide semiconductors, such as Shottky diodes and field effecttransistors (FET) having a palladium gate. The Shottky diode in thepresently preferred embodiment of the hydrogen analyzer of the inventionis capable of detecting hydrogen dissolved in water at concentrationsfrom about 0.1 nM to about 100 nM. It is theorized that greaterconcentrations of up to 1,000,000 nM are easily detectable usingdifferent hydrogen-sensing components.

[0031] In another aspect, the present invention provides processes formeasuring the amount of hydrogen in an aqueous solution including thesteps of: (1) equilibration of water containing dissolved hydrogen witha carrier gas; (2) removal of oxygen and, where present, carbonmonoxide, moisture and sulfur compounds from the carrier gas containinghydrogen; and (3) measuring the amount of hydrogen in the carrier gasthat has been treated to remove oxygen, carbon monoxide, moisture andsulfur compounds. Step 2 of the processes of the present inventionpreferably further includes removal of moisture from the carrier gascontaining hydrogen. The processes of the present invention neitherconsume nor produce hydrogen. Preferably a solid state sensor is used tomeasure the concentration of hydrogen. A presently preferred method ofmeasuring hydrogen concentration is by monitoring an output voltage fromthe solid state sensor and calculating the rate of voltage increase.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The foregoing aspects and many of the attendant advantages ofthis invention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

[0033]FIG. 1 shows a circuit diagram representing a presently preferredconfiguration of the hydrogen analyzer of the present invention.

[0034]FIG. 2 shows a cross-sectional view of a presently preferredembodiment of carbon monoxide and sulfur compound removal component 17.

[0035]FIG. 3 shows a circuit diagram representing a presently preferredembodiment of hydrogen sensor 7.

[0036]FIG. 4 shows a representative application of the hydrogen analyzerof the present invention to measure the concentration of dissolvedhydrogen in groundwater.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0037] The present invention provides apparatuses and processes for themeasurement of hydrogen in aqueous solution at concentrations as low asabout 0.1 nM. The apparatuses and processes of the present invention arecapable of accurately and reproducibly measuring the concentration ofdissolved hydrogen in an aqueous solution that may also contain otherdissolved gases, such as oxygen, carbon monoxide and sulfur compounds.

[0038]FIG. 1 shows a presently preferred configuration of a hydrogenanalyzer 38 of the present invention. Hydrogen analyzer 38 includes ahydrogen sensor 7, a mass transfer device 10, a gas reservoir 4, anoxygen removal cartridge 15, a moisture removal cartridge 16, and acarbon monoxide and sulfur compound removal cartridge 17. In a preferredembodiment of the present invention, the carbon monoxide and sulfurcompound removal cartridge 17 serves as the gas reservoir 4, in whichcase a separate reservoir is not required. In operation, watercontaining dissolved hydrogen, is equilibrated with a carrier gas bymeans of gas flow in a first flow circuit 11. Equilibrated carrier gaswithin gas equilibration volume 4 is then circulated through a secondflow circuit 14 during which oxygen, water, carbon monoxide, and sulfurcompounds are removed from the carrier gas without consuming orproducing hydrogen. Sensor 7 then measures the amount of hydrogen in thecarrier gas.

[0039] With reference again to FIG. 1, during the sensor preparationstage, called Stage A, air pump 1 pulls air in through first solenoidvalve 2 and second solenoid valve 3 and gas reservoir 4 via tubing 5.The air is then discharged from pump 1 through moisture removalcartridge 16, heated carbon monoxide and sulfur compound removalcartridge 17, third solenoid valve 6 and fourth solenoid valve 44, andhydrogen sensor 7 prior to being discharged to the atmosphere throughfifth solenoid valve 8. The purpose of Stage A is to allow residualhydrogen present on sensor 7 to be oxidized by oxygen present in air.Presence of residual hydrogen on sensor 7 is typically indicated by asensor output voltage that does not return to its baseline voltagefollowing exposure to hydrogen. Residual hydrogen on the sensor isespecially problematic in oxygen-free gas, such as is generated withinhydrogen analyzer 38 of the present invention.

[0040] The length of Stage A is dependent on the characteristics ofsensor 7. In the case of a metal-oxide semiconductor Shottky diode witha palladium/silver gate and an alumina insulator, a one-minute Stage Ais usually sufficient but may need to be longer. Preconditioning may berequired regularly or intermittently with specific sensors 7. Thesepreconditioning steps may include exposure to nitrogen or hydrogen atvarious temperatures for various times and will be dependent on thespecific nature of sensor 7.

[0041] With reference again to FIG. 1, during Stage B, which is referredto as the nitrogen purge stage and which follows Stage A, nitrogen gasis used to purge air and traces of hydrogen from oxygen removalcartridge 15, tubing 5, gas reservoir 4, moisture removal cartridge 16,and heated carbon monoxide and sulfur compound removal cartridge 17.Alternately, other methods of hydrogen removal can be utilized withinthe scope of the present invention, such as a thermal oxidizer. Thepurpose of Stage B is to remove traces of hydrogen that can interferewith quantification of low concentrations of dissolved hydrogen in anaqueous sample that is being analyzed. Nitrogen enters through sixthsolenoid valve 42 from cylinder 43 and passes through oxygen removalcartridge 15, second solenoid valve 3, gas reservoir 4, tubing 5, pump1, moisture removal cartridge 16, heated carbon monoxide and sulfurcompound removal cartridge 17, third solenoid valve 6 and fourthsolenoid valve 44, and sensor 7, and exits through fifth solenoid valve8.

[0042] Stage C follows Stage B and is referred to as the hydrogenequilibration stage during which hydrogen gas in the aqueous samplebeing analyzed is equilibrated with the carrier gas in gas reservoir 4and first flow circuit 11. With reference again to FIG. 1, in Stage Csixth solenoid valve 42 and fifth solenoid valve 8 close and thirdsolenoid valve 6 changes direction to promote flow in first flow circuit11 through air pump 1, cartridges 16 and 17, third solenoid valve 6, thegas side of mass transfer module 10, second solenoid valve 3, and gasreservoir 4. The aqueous sample being analyzed is pumped through theliquid side of gas transfer module 10 by pump 12 via tubing 13. Theduration of time allowed for Stage C is determined by the time requiredfor hydrogen in the aqueous sample being analyzed to equilibrate withthe carrier gas in gas reservoir 4 and first flow circuit 11. Stage Ctime is typically in the range of 1-10 minutes. The void volumes ofmoisture removal cartridge 16 and sulfur compound removal cartridge 17can serve as an equivalent gas reservoir 4, if desired.

[0043] Mass transfer module 10 can be any device that allowsequilibration of dissolved hydrogen with the carrier gas phase. Masstransfer module 10 is preferably constructed from hollow fiber gastransfer modules or sparging devices. These hollow fiber modules arecomposed of a plurality of hollow fiber membranes encased in a shellwith integral manifold. The manifold mechanically supports the ends ofthe hollow fiber membranes and directs carrier gas in and out of thelumens of the hollow fiber membranes. The shell surrounds the hollowfiber membranes and directs liquid water past the outer surface of thehollow fiber membranes. Well-designed modules will optimize liquid flowpatterns to minimize liquid phase mass transfer resistance.Additionally, well-designed modules will contain hollow fiber membranesthat have a high permeability for hydrogen. The sparging devices aredesigned similarly to a continuous flow gravity settler in which carriergas is introduced into a flowing liquid via a porous sparging element.

[0044] By way of non-limiting example, module GT-0204005 manufactured byNeoMecs (Eden Prairie, Minn.) contains 0.5 square feet of a coatedmicroporous hollow fiber. The gas permeability (P/l) of this membrane isapproximately 1×10⁻⁴cm³/cm²-sec-cmHg which is sufficient for the presentinvention. However, design of the liquid flow pattern is more criticalthan membrane permeability. Evaluation of the NeoMecs model GT-02010013module which has 0.13 square feet of the identical membrane wassubstantially inferior to the model GT-0204005 mainly because of theliquid flow design and not because of the difference in membrane surfacearea according NeoMecs. A mass transfer time of 1-10 minutes andpreferably 2-5 minutes is usually sufficient for equilibration ofdissolved hydrogen concentrations up to 10 nM where a NeoMecs GT-0204005module is used which has 0.5 square feet of membrane surface area and awater flow of 100-1000 milliliters per minute. Water flow is preferably300-1,000 milliliters per minute. Other mass transfer configurations aresuitable and could be readily evaluated by one of ordinary skill in theart without undue experimentation, such as the spargers noted above.

[0045] Stage D follows Stage C and is referred to as the carrier gaspreparation stage. During Stage D, oxygen, carbon monoxide, water, andsulfur compounds are removed from the carrier gas, With reference againto FIG. 1, in Stage D, second solenoid valve 3 and third solenoid valve6 reverse direction and fourth solenoid valve 44 and seventh solenoidvalve 45 adjust to promote carrier gas flow through second circuit 14while bypassing sensor 7. Gas flow through second circuit 14 entailsdischarge from pump 1 to moisture removal cartridge 16, heated carbonmonoxide and sulfur compound removal cartridge 17, third solenoid valve6, fourth solenoid valve 44 and seventh solenoid valve 45, oxygenremoval cartridge 15, second solenoid valve 3, gas reservoir 4, and backto air pump 2. During gas flow through second circuit 14, oxygen, carbonmonoxide, water, and sulfur compounds are removed from the carrier gasto levels such that detection of low levels of hydrogen in the carriergas is possible by hydrogen sensor 7.

[0046] Oxygen can be present in the carrier gas due to transfer ofoxygen, that is present in the aqueous solution being analyzed, acrossmass transfer module 10. For example, groundwater that is referred to as“anaerobic”, i.e., lacking oxygen, may not always be devoid of dissolvedoxygen. The presence of dissolved oxygen in “anaerobic” water is anunexpected observation. Traces of dissolved oxygen (i.e., less than 1milligram per liter) can be present in an aqueous solution, especiallyin groundwater, that is supporting an iron-reducing terminal electronaccepting process. This process employs iron-reducing bacteria that canbe facultative aerobes and thus can live in the presence of dissolvedoxygen. These traces of oxygen prevent sensor 7 from having the requiredsensitivity to low concentrations of hydrogen. This problem is mostnotable for iron-reducing terminal electron accepting processes wherethe typical dissolved hydrogen concentrations are very low, namely 0.1to 1.0 nM.

[0047] Oxygen is removed from the carrier gas by oxygen removalcartridge 15 during Stage D. Oxygen removal cartridge 15 removes oxygenfrom carrier gas that flows through an oxygen removal compositioncontained within the module. The oxygen removal composition withinoxygen removal cartridge 15 is preferably capable of efficiently andrapidly removing oxygen from the carrier gas without producing orconsuming hydrogen. The oxygen removal composition may be composed ofany material as long as it possesses the foregoing characteristics.Ascorbic acid-based preparations such as Anaeropack and Anaeropouchmanufactured by Mitsubishi Gas Chemical America, Inc. (New York, N.Y.)are the presently preferred oxygen removal compositions. Other oxygenremoval compositions useful in the practice of the present inventioninclude, but are not limited to: catalyzed ascorbic acid, alkalinehydroquinone or catechol, catalyzed hydroquinone or catechol, catalyzedsulfite, chelated salicylic acid, and catalyzed dicarboxylic acids.Preferably oxygen removal cartridge 15 will include a calorimetricoxygen indicator.

[0048] Oxygen absorbing compositions that are not useful as oxygenremoval compositions in the practice of the present invention include:sodium borohydride (produces hydrogen), lithium aluminum hydride(produces hydrogen), carbon monoxide-reduced, finely-dispersed cupricoxide catalyst (adsorbs hydrogen), hydrogen-reduced, finely-dispersedcopper catalyst (releases adsorbed hydrogen), iron powder-basedpreparations (not preferred because is slow and can produce hydrogen viacorrosion), heated copper (can produce hydrogen from water viacorrosion), lithium-based oxygen scavengers, such as Nanosorb resin(produces hydrogen from water), and zirconium sponge-based oxygenscavengers such as the High Capacity Gas Purifier by Supelco (produceshydrogen from water).

[0049] While catalyzed ascorbic acid or alkaline hydroquinonepreparations are useful as oxygen removal preparations for use incartridge 15, their reaction with oxygen results in an unexpected sidereaction that forms carbon monoxide. Removal of produced carbon monoxideis required and is accomplished by sulfur compound and carbon monoxideremoval cartridge 17. Carbon monoxide is removed so as to preventinterference with measurement of hydrogen by sensor 7. Carbon monoxideremoval is required only if generated during the process, such as whenusing the oxygen removal method of the preferred embodiment of theinvention, but may not be required for alternate embodiments of theinvention. Carbon monoxide can be present in anaerobic water thatcontains sulfidogenic or methanogenic bacteria and can be transferred tothe carrier gas across gas transfer module 10. For example, carbonmonoxide is generated by Methanosarcina barkeri during formation ofmethane and carbon dioxide from acetate where carbon monoxide is anintermediate (G. Gottschalk, Bacterial Metabolism, Springer-Verlag, NewYork. pp. 257-259 (1996)).

[0050]FIG. 2 shows a cross-sectional view of a presently preferreddesign of carbon monoxide removal cartridge 17. Carrier gas flows intoand out of carbon monoxide removal cartridge 17 via ports 46. Heatconducting tube 18 and end caps 19 are encased with insulation 20.Carbon monoxide removal composition 21 is contained within tube 18. Apreferred carbon monoxide removal composition is a finely dispersedcupric oxide catalyst named R3-11 and manufactured by BASF (Parsippany,N.J.). Metal oxide catalysts including those based on copper and zinc,are suitable for use as a carbon monoxide removal composition in thepractice of the present invention. Catalyst R3-11, in its oxidized form,is capable of removing carbon monoxide from carrier gas when heated.

[0051] Self-regulating heater 22 heats preparation 21 to the desiredtemperature when powered by power source 24 that is connected to heater22 by wires 23. The temperature control is significant becausesufficient carbon monoxide removal does not occur at temperatures thatare too low and hydrogen generation occurs due to corrosion processes attemperatures that are too high. Preferably the temperature should bebetween about 45° C. to 90° C., and preferably between about 55° C. andabout 80° C. for Catalyst R3-11. The ability of oxidized R3-11 to removecarbon monoxide at these temperatures was not expected based on vendorliterature that indicates a minimum temperature requirement of 100° C.Additionally, the vendor literature indicates that hydrogen will also beremoved at temperatures greater than 100° C. However, at temperaturesbetween 55° C. and 80° C. carbon monoxide is removed and hydrogen is notremoved.

[0052] The present inventor discovered that carbon monoxide is adsorbedto R3-11 but is not oxidized to carbon dioxide at temperatures less than100° C. Hydrogen is not adsorbed on oxidized R3-11 at these temperaturesand at room temperature. Interestingly, hydrogen is adsorbed by carbonmonoxide-reduced R3-11 at room temperature. These observations wereunexpected and resulted in oxidized R3-11 (as opposed to reduced R3-11)having great utility for removal of carbon monoxide in the presentinvention. Other catalysts useful as carbon monoxide removalcompositions in the practice of the present invention include, but arenot limited to, Carulite, formerly known as Hopcalite. Carulite is alow-temperature oxidation catalyst composed of manganese dioxide, copperoxide, and aluminum oxide and is manufactured by Carus Chemical Co.(LaSalle, Ill.). Preferably the carbon monoxide removal composition willinclude a calorimetric carbon monoxide indicator.

[0053] Metal oxide, carbon monoxide removal compositions, such as R3-11,when used for carbon monoxide removal in the manner described above,will have a finite lifetime because carbon monoxide is being adsorbedrather than oxidized to carbon dioxide. Eventually the binding sites forcarbon monoxide will become saturated and the preparation will have nofurther capacity for carbon monoxide removal. When saturation of theavailable binding sites occurs, the carbon monoxide removal compositionwill require replacement or regeneration. The presence of water vaporalso limits the lifetime of metal oxides that are used to remove carbonmonoxide. The likely reason is that the metal oxide composition is beingused at temperatures below the boiling point of water and thus watervapor condenses on the metal oxide surface and decreases the capacityfor carbon monoxide adsorption.

[0054] In the practice of the present invention, moisture removalcartridge 16 is preferably included in hydrogen analyzer 38, at a pointin first flow circuit 11 and second flow circuit 14 prior to carbonmonoxide removal cartridge 17, in order to absorb water from the gasentering cartridge 17, thereby minimizing the replacement frequency ofcarbon monoxide removal cartridge 17. Moisture removal cartridge 16 cancontain any moisture removal preparation as long as it does not consumeor generate hydrogen and removes sufficient moisture to preventpremature limitation of the lifetime of carbon monoxide removalcartridge 17. A presently preferred moisture-removal composition forinclusion in moisture removal cartridge 16 is 13X molecular sieves.Calcium sulfate preparations, such as Drierite (WA Hammond DrieriteCompany, Ltd., Xenia, Ohio), are also useful. Preferably, a calorimetricmoisture indicator, such as CoCl₂, is included in moisture removalcartridge 16. Placement of moisture removal cartridge 16 in the line ofgas flow following oxygen removal cartridge 15 and preceding carbonmonoxide removal cartridge 17 is required because certain compositionsincluded in oxygen removal cartridge 15 may release moisture that mustbe removed from the carrier gas by moisture removal cartridge 16 priorto entering carbon monoxide removal cartridge 17.

[0055] Sulfur compounds such as hydrogen sulfide can also interfere withmeasurement of hydrogen by sensor 7. Sulfur compounds, when present, aremore problematic than oxygen and carbon monoxide because they can poisonsensor 7 and disable it from further use. One sulfur compound, hydrogensulfide, is common in anaerobic water because sulfidogenic bacteria canproduce it from sulfate. Because hydrogen sulfide is volatile, it istransferred across gas transfer module 10 into the carrier gas.

[0056] Sulfur compounds are preferably removed from the carrier gas bycarbon monoxide removal cartridge 17 during Stage D. Any compositionthat is capable of adsorbing sulfur compounds can be included in carbonmonoxide removal cartridge 17 so long as it does not consume or producehydrogen and removes sulfur compounds sufficiently so as to preventpoisoning of sensor 7. Copper oxide catalysts, such as R3-11, arepresently preferred for sulfur compound removal and thus serve a dualpurpose in the present invention: they can remove carbon monoxide andsulfur compounds simultaneously. Other preferred sulfur adsorbingcompounds include, but are not limited to: BASF catalyst R3-12(Parsippany, N.J.) which includes zinc oxide as well as copper oxide,and hydrated iron oxide. Preferably, a colorimetric sulfur compoundindicator is included in carbon monoxide removal cartridge 17.

[0057] The duration of Stage D depends, in part, on the identity of thespecific compositions used in oxygen removal cartridge 15, moistureremoval cartridge 16, and carbon monoxide removal cartridge 17. Based onthe presently preferred configuration of hydrogen analyzer 38 of thepresent invention, the time period for removal of oxygen, sulfurcompounds, water, and carbon monoxide from the carrier gas is preferablyfrom about 0.5 minutes to about 10 minutes, more preferably from about 2minutes to about 4 minutes. Oxygen concentrations should be reduced toless than 0.1% v/v prior to proceeding to the next stage. Carbonmonoxide concentrations should be reduced to less than 10 ppm in thecarrier gas prior to proceeding to the next stage.

[0058] Stage E, referred to as the measurement stage, is the final stageduring which hydrogen is measured by hydrogen sensor 7. With referenceagain to FIG. 1, fourth solenoid valve 44 and seventh solenoid valve 45reverse direction to promote flow past sensor 7 in second flow circuit14. Sensor 7 is operated by an electrical circuit shown in FIG. 3 duringStages A, B, C, D, and E and data are recorded from sensor 7 duringStage E. FIG. 3 is a schematic representation of sensor 7 and associatedelectronics that are required for operation of sensor 7 in the presentlypreferred embodiment of hydrogen analyzer 38 of the present invention.As shown in FIG. 3, a Shottky diode 28 operates as the hydrogen-sensingcomponent of hydrogen sensor 7 in the presently preferred embodiment ofhydrogen analyzer 38 of the present invention. Any device that issensitive to hydrogen at the desired concentrations can be used as thehydrogen-sensing component of hydrogen sensor 7. Another example of auseful hydrogen-sensing device is a field effect transistor (FET) with apalladium gate.

[0059] In the presently preferred embodiment, sensor 7 is manufacturedusing silicon-based microfabrication technology on silicon semiconductorsubstrate 25 and includes platinum resistance heater 26, platinumresistance temperature detector (RTD) 27, and Shottky diode 28, with ahydrogen-sensitive gate. The Shottky diode is ametal-insulator-semiconductor (MIS) diode with p-type siliconsemiconductor, a two-part insulator composed of silica adjacent to thesemiconductor and alumina adjacent to the gate, and a metalpalladium/silver gate. Substrate 25, including platinum resistanceheater 26, platinum resistance temperature detector (RTD) 27, andShottky diode 28, is manufactured by Case Western Reserve University.Substrate 25, including platinum resistance heater 26, platinumresistance temperature detector (RTD) 27, and Shottky diode 28 are allenclosed in a TO-5 housing. The temperature of sensor 7 is controlled bycontroller 29 that takes a temperature input from RTD 27 as its processvariable and produces an output directed to heater 26 as its controlvariable. The temperature of sensor 7 is preferably maintained betweenabout 100° C. and about 200° C. The lower limit of the temperature rangeis selected to prevent moisture condensation. The upper limit of thetemperature range is selected based on the specific characteristics ofsensor 7. Shottky diode 28 is integrated into a circuit comprising powersource 30, voltmeter 31, and resistor 32.

[0060] Hydrogen dissolved in water is detectable with the presentlypreferred embodiment of hydrogen analyzer 38 down to concentrations ofabout 0.1 nM using the Shottky diode type sensor 7 described herein.This sensor is especially suited for detection of low hydrogenconcentrations but can be used to measure dissolved hydrogenconcentrations up to about 100 nM. Hydrogen analyzer 38 of the presentinvention can also be readily modified to detect higher concentrationsof dissolved hydrogen.

[0061] To the best of the inventor's knowledge, output from the Shottkydiode in the present invention is utilized in a novel manner. Typicallysolid state sensors for hydrogen yield a steady state output voltagethat is directly related to the hydrogen concentration. However, in thepresence of trace levels of hydrogen contained in a carrier gas devoidof oxygen, these sensors have been observed to be incapable of attaininga steady state voltage output within a reasonable timeframe. Rather theoutput increases continuously and the rate at which this outputincreases is linearly related to the hydrogen concentration. The reasonsfor this unexpected observation appear to be kinetic and diffusionallimitations that occur at low hydrogen concentrations.

[0062] Thus, during Stage E, the output from sensor 7 is monitored overa period of time and the rate of voltage increase is calculated. Thisrate is compared to a calibration curve in order to quantify thehydrogen concentration. For example, a concentration of 0.1 ppm hydrogenyields a rate of approximately 10 millivolts per minute, a concentrationof 1.0 ppm hydrogen yields a rate of approximately 100 millivolts perminute, and a concentration of 10 ppm hydrogen yields a rate ofapproximately 1,000 millivolts per minute. The carrier gas hydrogenconcentrations can then be related to dissolved hydrogen concentrationsin water by Henry's law coefficients (R. H. Perry and C. H. Chilton,Chemical Engineers' Handbook, McGraw-Hill Book Company, New York, P.3-97. (1973)) or Ostwald coefficients (P. Gerhardt, R. G. E. Murray, W.A. Wood, and N. R. Krieg, Methods for General and MolecularBacteriology, American Society for Microbiology. Washington D.C., pp.145 and 184 (1994)). For example, at 20° C., Henry's coefficient forhydrogen in water is 6.83×10⁴ atmospheres. The dissolved hydrogenconcentration in water is calculated by the following equation:

C _(L)=10⁹ P _(A) C _(G) C _(W)/H

[0063] where C_(L) is the dissolved hydrogen concentration in liquidwater in units of nM, 10⁹ is for conversion from units of M to nM, P_(A)is the ambient pressure in units of atmospheres (typically equal to 1),C_(G) is the hydrogen concentration measured in the carrier gas in unitsof ppm, C_(W) is the concentration of pure water in molarity (M) unitsand is equal to 55.6 M, and H is Henry's constant in units ofatmospheres. H is temperature-dependent and at 20° C. this equation canbe reduced for most applications to:

C _(L)=0.81C _(G)

[0064] At 10° C. the equation can be reduced to:

C _(L)=0.87C _(G)

[0065] Finally, FIG. 4 depicts field application of hydrogen analyzer 38of the present invention for measurement of dissolved hydrogen ingroundwater. Contaminated ground 33 includes groundwater at level 34 anda groundwater monitoring well 35. Groundwater is pumped through tubing36 via pump 37 to dissolved hydrogen analyzer 38 which discharges towaste container 39. Pump 37 and dissolved hydrogen analyzer 38 arepowered by a 12-volt battery in automobile 40 via jumper cables 41.Typically, the total time required to complete Stages A-E and obtain ameasurement of the concentration of dissolved hydrogen is approximately15-20 minutes.

[0066] Hydrogen analyzer 38 of the present invention has numerous uses.In general, the concentration of dissolved hydrogen can be used tomonitor the nature, extent, rate, or stability of anaerobic biologicalsystems.

[0067] Natural attenuation, also known as passive remediation orintrinsic remediation, is based on the natural ability of microorganismspresent in groundwater to biodegrade environmental contaminants.Different microorganisms are capable of mediating these biodegradationprocesses. One method of classification of these bacteria is by the typeof terminal electron accepting process (TEAP) used during oxidation oforganic contaminants. For example, the aerobic TEAP is employed bybacteria using oxygen as the terminal electron acceptor. Different TEAPsare employed by anaerobic bacteria. Anaerobic TEAPs includedenitrification, manganese reduction, ferric iron reduction,sulfidogenesis, and methanogenesis. Knowledge of which TEAPs exist in agiven aquifer is important with respect to understanding biodegradationprocesses that are occurring.

[0068] Dissolved hydrogen concentration is an indicator of the type ofTEAPs that are present in a body of water. Dissolved hydrogenconcentrations ranging from 0.1-1.0 nM are associated with theiron-reducing TEAP, dissolved hydrogen concentrations ranging from 2-6nM with the sulfidogenic TEAP, and dissolved hydrogen concentrationsranging from 10-20 nM with the methanogenic TEAP (F. H. Chapelle and P.B. McMahon, J. Hydrology, 127:85-108 (1991)). One application of thepresent invention is to measure dissolved hydrogen concentrations ingroundwater at contaminated sites. Measurement of dissolved hydrogenallows determination of the types of TEAPs that are present, and thusprovides information about specific biodegradation processes. Forexample, reductive dechlorination is one mechanism of biodegradation ofchlorinated organic contaminants. These contaminants can includetrichloroethene (TCE) among others. Reductive dechlorination of TCE viaa biological mechanism can involve the pathway: TCE→cDCE→VC→CO₂ wherecDCE is cis-dichloroethene, VC is vinyl chloride, and CO₂ is carbondioxide. The first two reactions (TCE→cDCE→VC) are based on reductivedechlorination and typically occur in methanogenic and sulfidogenicTEAPs. The last reaction (VC→CO₂) is known to occur in the iron-reducingTEAP. Knowledge of which TEAPs exist in different zones of an aquifercan indicate which of these reactions are occurring at a site.

[0069] Enhancement of reductive dechlorination can also be accomplishedby increasing or adjusting the dissolved hydrogen concentration ingroundwater to a desired level. Dissolved hydrogen can be adjusted bysparging a mixture of hydrogen in nitrogen into groundwater therebyincreasing the dissolved hydrogen concentration. Dissolved hydrogen canalso be adjusted by adding the lactic acid-based preparation known asHydrogen Release Compound (HRC) manufactured by Regenesis (San JuanCapistrano, Calif.). Lactic acid is slowly released by this preparationand is subsequently consumed by anaerobic bacteria that generatehydrogen. No matter how dissolved hydrogen in groundwater is adjusted, ameans for measurement is required. The present invention can be used tomeasure dissolved hydrogen in such an application and thus facilitateaccurate hydrogen concentration adjustment.

[0070] Anaerobic digesters are used to biodegrade waste water containingvarious organic compounds. These digesters employ consortia of anaerobicbacteria to accomplish the overall reaction: Organic matter →CO₂+CH₄where CO₂ is carbon dioxide and CH₄ is methane. Instability of theseconsortia and thus of the digestor is a major problem in waste watertreatment. Digestors commonly become unstable and “go sour” which isattributable to excess acid production. Measurement of pH is not auseful process variable for controlling digestor stability because itdoes not indicate instability at a sufficiently early point in time.Dissolved hydrogen concentration is a better indicator of digestorstability and is useful as a process variable to be used in a processcontrol algorithm. Dissolved hydrogen has been described as an “idealvariable” for monitoring and control of anaerobic systems (J. -C. Frigonand S. R. Guiot, Enzyme Microb. Technol., 17:1080-1086 (1995)).Nonetheless, the dissolved hydrogen may be present at concentrationsless than 100 ppm (W. R. Slater, M. Merigh, N. L. Ricker, F. Labib, J.F. Ferguson, and M. M. Benjamin, Wat. Res., 24:121-123 (1990)). Thepresent invention, which is capable of sensing dissolved hydrogen downto a concentration of about 0.1 nM, is thus useful for monitoringdigestor stability.

[0071] Many useful biochemical products are produced in bioreactors thatemploy microorganisms. Many of these bioreactors are operatedanaerobically. Examples of biochemical products produced in bioreactorsthat employ microorganisms include antibiotics, amino acids, proteins,vitamins, growth regulators, hormones, steroids, beer, and wine. Controlof these processes is often difficult because the process variables arenot easily measured. For example, antibiotics are sometimes measuredusing a bioassay that requires an inordinate amount of time forcompletion. Such assays are not amenable to incorporation into processcontrol strategies. Surrogate process variables are of interest in thebiochemical process industry and include pH, carbon dioxide evolutionrate, and carbon source concentration, among others. These surrogateprocess variables are incorporated into a process control algorithm toenable prediction of biochemical reaction extent or rate. Dissolvedhydrogen is another process variable that is useful in this regard andhas been used to evaluate antibiotic susceptibility (E. G. Hornsten, H.Elwing, E. Khilstrom, and I. Lundstrom, J. Antimicrobial Therapy,15:695-700 (1985)) and bioreactor mixing inhomogeneity (N. Cleland, E.G. Horsten, H. Elwinng, S. -O. Enfors, and I. Lundstrom, Appl.Microbiol. Biotechnol., 20:268-270 (1984)). Pd-MOS sensors have beenused for these applications but may not be applicable in cases wherehydrogen sulfide, carbon monoxide, or oxygen are present. In thesecases, hydrogen analyzer 38 of the present invention will be useful.

[0072] Subsurface permeable walls composed of iron filings or othermetallic materials are a useful remediation tool for chlorinated solventplumes in aquifers. These metal-reactive walls promote reductivedechlorination by anaerobic corrosion processes that produce hydrogen.The rates of corrosion and reductive dechlorination are related to therate of hydrogen production (E. Reardon, Environ. Sci. Technol.,29:2936-2945 (1995)). Dissolved hydrogen concentrations are apotentially important parameter that can be used to monitor the rate orextent of such processes. These metal-reactive walls are in effectconsumed over time because of corrosion. In addition, the rate candecrease due to passivation of the metal surfaces. Thus measurement ofdissolved hydrogen can also be used to monitor the status of thesemetal-reactive walls. Upon a significant decrease in dissolved hydrogenconcentration, for example, the metal-reactive wall may be in need ofregeneration or replacement. The present invention will be useful formaking these determinations.

[0073] Dissolved hydrogen can be used as an indicator of corrosion.Hydrogen is formed from water when metals undergo corrosion in theabsence of oxygen. Hydrogen can also cause embrittlement stress crackingcorrosion (SCC) of stainless steels. Measurement of dissolved hydrogento assess and monitor corrosion is another use for the presentinvention. Monitoring of corrosion pertains to operation of processequipment, pipelines, boilers, and any metal system that involves theprocessing, storage, or conveyance of water-based liquids.

[0074] While the presently preferred use of the present invention ismeasurement of dissolved hydrogen in aqueous solution, measurement ofdissolved hydrogen in nonaqueous liquids such as transformer oil ispossible.

[0075] The following examples merely illustrate the various embodimentsnow contemplated for practicing the invention, but should not beconstrued to limit the invention.

EXAMPLE 1 Dissolved Hydrogen Transfer to Gaseous Nitrogen Across NeoMecsGT-0204005 Hollow Fiber Membrane Module

[0076] Groundwater containing different concentrations of dissolvedhydrogen was circulated through a NeoMecs GT-0204005 hollow fiber gasmembrane module. This module contains 0.5 square feet of membranesurface area. The water containing dissolved hydrogen was pumped throughthe shell of the module at a flow rate of 220 milliliters per minute. Avolume of 12 milliliters of nitrogen gas was circulated through thelumen of the hollow fibers at a flow rate of 5,000-7,000 milliliters perminute.

[0077] The concentration of dissolved hydrogen in the water wasdetermined using the bubble strip method in conjunction with a reductiongas analyzer (F. H. Chapelle and P. B. McMahon, J. Hydrology, 127:85-108(1991)). This concentration is reported in Table I as C_(G), theequilibrated gas-phase hydrogen concentration in ppm. Table I shows thatan equilibrated gas-phase hydrogen concentration of 0.43 ppm (equivalentto 0.34 nM dissolved hydrogen) equilibrated across the hollow fibermembrane module in 4 minutes. An equilibrated gas-phase hydrogenconcentration of 87 ppm (equivalent to 70 nM dissolved hydrogen)equilibrated across the hollow fiber membrane module in 6 minutes. TABLEI Gas-Phase Hydrogen Concentration (ppm) Transferred Across NeoMecsGT-0204005 Hollow Fiber Membrane Module Bubble Strip HydrogenConcentration C_(G) Time (minutes) (ppm) 0 2 4 6 0.43 0.15 0.36 0.41 —87 1.5 47 72 90

EXAMPLE 2 Hydrogen Sensor Response (mV/min) in Presence of Oxygen orCarbon Monoxide in Nitrogen Carrier Gas

[0078] A Shottky diode MIS hydrogen sensor containing an aluminainsulator and a palladium/silver metal gate was fabricated and testedfor hydrogen sensitivity in nitrogen containing oxygen and carbonmonoxide. The sensor temperature was controlled at 143° C. for thesetests. Table II shows the response of the sensor to hydrogen in nitrogenas Test 1. The response was significantly lower in the presence of lowconcentrations of oxygen (Test 2) or carbon monoxide (Test 3). TABLE IIHydrogen Sensor Response (mV/min) in Presence of Oxygen and CarbonMonoxide in Nitrogen Carrier Gas Gas Phase Hydrogen Concentration (ppm)Test Conditions 0.1 1 10 1 Control 49 252 2173 2 0.5% O₂ 2.9 38.2 546.53 0.1% CO −6.3  34.8  87.9

EXAMPLE 3 Removal of Oxygen and Carbon Monoxide from Carrier Gas

[0079] Oxygen and carbon monoxide removal from nitrogen carrier gas wastested with different compositions capable of removing oxygen and carbondioxide, and gaseous hydrogen concentrations were monitored. Theascorbic acid-based preparation named Anaeropack manufactured byMitsubishi Gas Chemical Corporation (MGC) America, Inc. (New York, N.Y.)was used for oxygen scavenging and the copper oxide-based preparationnamed Catalyst R3-11 manufactured by BASF Corporation was used forcarbon monoxide scavenging. Anaeropack was used as purchased and wasplaced in a plastic module through which carrier gas flowed. CatalystR3-11 was used as purchased and was placed in a brass pipe through whichcarrier gas flowed and heated to a constant temperature of 70° C. A500-milliliter volume of nitrogen carrier gas containing oxygen wascirculated and gas samples were collected and analyzed on a gaschromatograph for oxygen and on a reduction gas analyzer for hydrogenand carbon monoxide.

[0080] Table III shows results for a control test (Test 1) where noscavengers were used, a second test (Test 2) where only the oxygenscavenger was used, and a third test (Test 3) where both scavengers wereused. The results show that oxygen was not removed in Test 1. Oxygen wasremoved to 0% in Test 2 and hydrogen did not increase significantly. Thecarbon monoxide concentration did increase significantly due to reactionof oxygen with the oxygen scavenging preparation. Test 3 shows that inthe presence of both scavengers oxygen and carbon monoxide were bothremoved. During Test 3 hydrogen did not increase significantly. TABLEIII Removal of Oxygen and Carbon Monoxide from Carrier Gas ConcentrationCarbon Mon- Oxygen (%) Hydrogen (ppm) oxide (ppm) Test ConditionsInitial Final Initial Final Initial Final 1 Control 2.6 2.9 1.87 1.971.2 1.3 2 O₂ 2 0 0.23 0.25 1.6 15 Scavenger 3 O₂ & CO 2 0 0.18 0.21 0.650 Scavengers

EXAMPLE 4 Effect Of Water On Carbon Monoxide Removal by Catalyst R3-11

[0081] The effect of water on carbon monoxide removal by copperoxide-based Catalyst R3-11 was tested. A quantity of R3-11 was placed ina steam autoclave and treated for 15 minutes at 121° C. to saturate thecatalyst with water vapor. The catalyst was then placed in a brassmodule through which gas flowed and was heated to 70° C. A 10-millilitervolume of nitrogen carrier gas was circulated through the heatedcatalyst and 0.1 milliliter of carbon monoxide was injected into thecarrier gas. Carbon monoxide concentration was monitored over time.Table IV shows that carbon monoxide concentration did not decreasesignificantly over four minutes. A control test where new Catalyst R3-11was used and was not treated in the autoclave demonstrated significantremoval of carbon monoxide from the carrier gas in less than fourminutes as shown in Table IV. TABLE IV Effect of Water on CarbonMonoxide Concentration (ppm) in Presence of Catalyst R3-11 Time(minutes) Condition 0 2 2.5 4 Control 2,900 14 — 1.3 Water 1,200 — 610530

[0082] In practice, water removal from carrier gas is preferred becauseof humidity that is created during equilibration of groundwater with thecarrier gas in the hollow fiber membrane module, and because theascorbic acid-based oxygen removal preparation is moist and releasesmoisture during use. Use of drying agents such as molecular sieves haveproven to be useful in this regard. Tests using Catalyst R3-11 incombination with 13X molecular sieves to remove water from carrier gasshow that carbon monoxide removal is sustained for at least 20 cycles ofoxygen removal. Use of Catalyst R3-11 in the absence of 13X molecularsieves results in dramatic reduction of carbon monoxide removalcapability within two to three cycles.

EXAMPLE 5 Analysis of Groundwater Containing Dissolved Hydrogen

[0083] Groundwater containing dissolved hydrogen was analyzed usingdissolved hydrogen analyzer 38 depicted in FIG. 1 and results werecompared to the bubble strip/reduction gas analyzer method. Thegroundwater was removed from a former manufacturing facilitycontaminated with chlorinated hydrocarbons. cis-1,2-Dichloroethene(cDCE) and vinyl chloride (VC) are the predominant contaminants presentin groundwater. Groundwater samples were collected using a pneumaticbladder pump at a volumetric flow rate of 1,000 milliliters per minute.The actual dissolved hydrogen concentrations were as shown in Table V.Hydrogen analyzer 38 was operated with Stage A (sensor preparation) for2 minutes, Stage B (nitrogen purge) for 1 minute, Stage C (hydrogenequilibration) for 3 minutes, Stage D (carrier gas preparation), andStage E (measurement). Catalyst R3- 11 was operated at 70° C. The sensorwas operated at 117° C. for these tests. Table V shows analyzer 38 wascapable of measuring dissolved hydrogen with a percent error rangingfrom −6.2% to 14%. TABLE V Hydrogen Analyzer Results with GroundwaterSamples Hydrogen Concentration (nM) Test Actual Analyzer Difference (%)1 1.4 1.6 14.3% 2 0.96 1 4.2% 3 0.96 0.9 −6.2%

[0084] While the preferred embodiment of the invention has beenillustrated and described, it will be appreciated that various changescan be made therein without departing from the spirit and scope of theinvention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A hydrogen analyzercomprising in gaseous communication: a mass transfer unit, having aliquid portion and a gaseous portion, through which hydrogen gas istransferred from a liquid analyte to a carrier gas; a gas equilibriumreservoir within which hydrogen gas transferred from the analyte isequilibrated; an oxygen unit for removal of oxygen from the carrier gascontaining hydrogen; a hydrogen sensor for measuring the amount ofhydrogen in the carrier gas from which oxygen have been removed; and apump for moving the carrier gas through the mass transfer unit, gasequilibrium reservoir, oxygen unit and hydrogen sensor, all of which areconnected in fluid flow communication.
 2. The hydrogen analyzer of claim1 , further comprising a carbon monoxide unit for removal of carbonmonoxide from the carrier gas containing hydrogen.
 3. The hydrogenanalyzer of claim 2 , further comprising a sulfur unit for removingsulfur compounds from the carrier gas containing hydrogen.
 4. Thehydrogen analyzer of claim 3 , wherein the sulfur and carbon monoxideremoval units comprise a unit including a composition that is capable ofremoving both carbon monoxide and sulfur compounds from the carrier gas.5. The hydrogen analyzer of claim 4 , wherein the gas equilibriumreservoir is defined by the sulfur and carbon monoxide removal unit. 6.The hydrogen analyzer of claim 4 , wherein the carbon monoxide andsulfur compound removal composition is catalyst R3-11.
 7. The hydrogenanalyzer of claim 6 , further comprising a heater coupled to heat thecarrier gas containing hydrogen to a temperature of 55° C. to 80° C. asthe carrier gas flows through the carbon monoxide unit.
 8. The hydrogenanalyzer of claim 2 , further comprising a heater coupled to heat thecarrier gas containing hydrogen as the carrier gas flows through thecarbon monoxide unit.
 9. The hydrogen analyzer of claim 1 , furthercomprising a sulfur unit for removing sulfur compounds from the carriergas containing hydrogen.
 10. The hydrogen analyzer of claim 1 , whereintreatment of the carrier gas containing hydrogen neither produces norconsumes hydrogen.
 11. The hydrogen analyzer of claim 1 , furthercomprising a moisture removal unit for removing moisture from thecarrier gas.
 12. The hydrogen analyzer of claim 1 , wherein the moistureremoval unit further comprises a moisture-removal composition selectedfrom the group consisting of molecular sieves and a calcium sulfatepreparation.
 13. The hydrogen analyzer of claim 1 , wherein the masstransfer unit is selected from the group consisting of a hollow fibergas transfer module and a sparger.
 14. The hydrogen analyzer of claim 1, wherein the oxygen removal unit comprises an oxygen removalcomposition.
 15. The oxygen removal component of claim 14 , wherein theoxygen removal unit is an ascorbic acid derivative.
 16. The hydrogenanalyzer of claim 1 , wherein the hydrogen sensor is selected from thegroup consisting of a Schottky diode and a field effect transistor. 17.The hydrogen analyzer of claim 1 , wherein the hydrogen sensor comprisesa metal oxide semiconductor.
 18. The hydrogen analyzer of claim 1 ,wherein the hydrogen sensor is capable of detecting hydrogen dissolvedin an aqueous medium at a concentration of on the order of 0.1 nM.
 19. Ahydrogen analyzer comprising in gaseous communication: a mass transferunit, having a liquid portion and a gaseous portion, through whichhydrogen gas is transferred from a liquid analyte to a carrier gas; agas equilibrium reservoir within which hydrogen gas transferred from theanalyte is equilibrated; a carbon monoxide unit for removal of carbonmonoxide from the carrier gas containing hydrogen; an oxygen unit forremoval of oxygen from the carrier gas containing hydrogen; a sulfurunit for removing sulfur compounds from the carrier gas containinghydrogen; a moisture unit for removing moisture from the carrier gas; ahydrogen sensor for measuring the amount of hydrogen in the carrier gasfrom which carbon monoxide, oxygen, sulfur compounds and moisture havebeen removed; and a pump for moving the carrier gas through the masstransfer unit, gas equilibrium reservoir, carbon monoxide, oxygen,sulfur and moisture units and hydrogen sensor, all of which areconnected in fluid flow communication.
 20. A process for measuring theamount of dissolved hydrogen in a solution comprising the steps of: (a)equilibration of liquid containing dissolved hydrogen with a carriergas; (b) removal of oxygen from the carrier gas containing hydrogen; and(c) measuring the amount of hydrogen in the carrier gas that has beentreated to remove oxygen.
 21. The process of claim 20 , wherein saidremoval step neither consumes nor produces hydrogen.
 22. The process ofclaim 20 , wherein step (6) further comprises removal of carbon monoxidefrom the carrier gas continuing hydrogen.
 23. The process of claim 20 ,wherein step (b) further comprises removal of sulfur compounds from thecarrier gas containing hydrogen.
 24. The process of claim 20 , whereinstep (b) further comprises removal of moisture from the carrier gascontaining hydrogen.
 25. The process of claim 20 , wherein a metal oxidesemiconductor is used to measure the concentration of hydrogen.
 26. Theprocess of claim 25 wherein hydrogen concentration is measured bymonitoring an output voltage from the metal oxide semiconductor andcalculating the rate of voltage increase.
 27. The process of claim 20 ,wherein step (b) farther comprises the removal of carbon monoxide at atemperature of 55° C. to 80° C.
 28. The process of claim 20 , wherein instep (a) the liquid containing dissolved hydrogen is an aqueous sampleof contaminated groundwater, further comprising a step (d) ofdetermining the status of bioremediation using the hydrogen contentmeasured in step (c).
 29. The process of claim 28 , wherein step (d)comprises monitoring hydrogen content to determine the status of naturalattenuation of contaminants.